Method of securing a mesh to a metal substrate for a bone implant

ABSTRACT

The wire mesh is welded to the metal substrate via step-like protuberances formed on the outer surface of the substrate. The wire mesh is permanently deformed in the regions over the protuberances to facilitate forming of spot welds while the remainder of the mesh outside the zones of the protuberances remains porous for the intake of bone cement or the ingrowth of bone tissue.

This is a division of application Ser. No. 07/308,244, filed Feb. 8,1989, now U.S. Pat. No. 4,969,904.

This invention relates to a method of securing a mesh to a metalsubstrate for a bone implant.

Heretofore, various types of bone implants has been known. For example,U.S. Pat. No. 4,660,755 describes a bone implant, for example for use asa hip prosthesis, wherein a substrate material is adapted to transmitloads imparted to a skeletal structure while a porous surface isprovided on the substrate to enhance biological fixation to a skeletalstructure. In order to construct the implant, the substrate and porouslayers are placed within a device whereby the porous layers may betightly compacted toward the substrate while current from a currentgenerator is conducted through the porous layers and substrate in orderto generate sufficient heat to cause a surface metallurgical bondingbetween the porous layers as well as between the porous layers and thesubstrate.

European Patent Application 0189546 also describes a bone implant inwhich a metal substrate serves as a support for a multilayer braided andpermanently deformable wire mesh having layers connected to one anotherand to the substrate by local weld zones.

French Patent 2548533 and German Gebrauchsmuster 8600970 describe boneimplants using wire mesh structures which are secured to the substrate.

However, it has been found that fixing mesh layers to one another and toa substrate by spot welds at discrete places is expensive and difficult.

Accordingly, it is an object of the invention to simplify the fixationof a wire mesh to a metal substrate in the formation of a bone implant.

It is another object of the invention to improve the securement of awire mesh to a metal substrate for a bone implant.

Briefly, the invention provides a method of securing a multilayer metalwire mesh to a metal substrate which includes the steps of providing asubstrate with a plurality of steplike protuberances on an outersurface, positioning a multilayer metal mesh over the surface andprotuberances of the substrate, pressing the mesh against the substrateto permanently deform the sections of the mesh which are disposed overthe protuberances without permanently deforming the remainder of themesh and welding the permanently deformed sections to the protuberancesand the layers of the deformed sections to each other. In accordancewith the method, the pressing of the mesh against the substrate and thewelding of the permanently deformed sections are performedsimultaneously and produce spot welds between the mesh layers andbetween the protuberances and the deformed sections of mesh.

When the wire mesh is pressed onto the substrate, the layers of the meshadapt to the contour of the substrate by permanent deformation. However,in the remaining regions of the substrate surface, there is only slight,if any, compression of the mesh structure. Because the discrete layersof the mesh experience substantial changes in shape and substantialcompression over the protuberances of the substrate, a relatively largecontact area is formed between the substrate and the mesh as well asbetween the individual mesh layers. Consequently, welding of theselayers, for example, using spot-welding electrodes permits theelectrodes to encounter an adequate metal bridge along which a "strand"of liquid metal is produced. This strand subsequently hardens into aweld joint to secure the contiguous layers of the mesh together as wellas the mesh to the protuberances. Advantageously, the spot-weldingelectrodes may be disposed in zones of a mesh-compressing ram used forthe pressing of the mesh against the substrate.

These and other objects and advantages of the invention will become moreapparent from the following detailed description taken in conjunctionwith the accompanying drawings wherein:

FIG. 1 diagrammatically illustrates a cross sectional view of a wiremesh in position for pressing against a substrate by a ram: and

FIG. 2 illustrates a cross sectional view of wire mesh being pressed andwelded to a substrate in accordance with the invention.

Referring to FIG. 1, the bone implant is to be formed of a metalsubstrate 1, for example, a metal shank for a hip joint prosthesis, anda multilayer metal Wire mesh 2.

The mesh 2 is in the form of a number of layers, the mesh size of whichdiffer and decrease from the outside toward the inside.

In order to receive the mesh 2, the substrate 1 has a steppedcross-section wherein the innermost surface provides a foundationsurface 3 for the mesh 2. In addition, a plurality of step-likeprotuberances 4 project from the foundation surface 3 and are disposedat places where the wire mesh 2 is to be connected to the substrate 1 byspot welds 5 as indicated in FIG. 2 as well as where the mesh layers areto be connected to one another.

Referring to FIG. 2, the substrate 1 may be in form of a shank while thewire mesh 2 is disposed over opposite sides of the substrate 1. Inaddition, the protuberances 4 are spaced along the foundation surface 3which acts as an outer surface of the substrate 1. In order to securethe mesh 2 to the substrate 1, the mesh 2 is pressed against thesubstrate 1 by means of a ram 6 in which welding electrodes 7 aredisposed. Upon pressing of the mesh 2 against the substrate 1, thesections of the mesh 2 disposed over the protuberances 4 permanentlydeform substantially without permanent deformation of the remainder ofthe mesh 2. That is, only slight compression substantially withoutpermanent deformation occurs in the zones of the foundation surface 3between the protuberances 4. Consequently, adequately compressedmaterial for the formation of spot welds 5 is available while, on theother hand, the porosity of the mesh 2 which assists the ingrowth ofbone tissue or the taking in of bone cement is not excessively impaired

During pressing in of the mesh, the welding electrode 7 may be activatedto generate a current sufficient to form spot welds 5 between theindividual layers of the mesh 2 as well as between the mesh 2 and theprotuberances 4 of the substrate 1. In this respect, the position of theprotuberances 4 and the position of the electrodes 7 are coordinatedwith one another so that the welds occur as spot welds 5 on theprotuberances 4.

As indicated in FIG. 2, where the substrate 1 forms a shank of a hipjoint implant, the mesh 2 may be disposed on opposite sides of theshank. In this respect, the mesh may be applied as individual layersonly to two opposite sides of the shank or may be in the form of a meshwhich is circumferentially disposed around the shank.

Of course, the substrate may be of any other shape and construction toreceive a wire mesh for use other than as a shank.

The invention thus provides a bone implant wherein a multilayer metalwire mesh is securely fixed to a substrate in a secure manner whileproviding sufficient porosity for the ingrowth of bone tissue forimplantation in a bone cement bed.

The invention further provides a technique for easily securing a metalwire mesh to a metal substrate. In this respect, the compression andpermanent deformation of the wire mesh at the protuberances of thesubstrate produces accumulations of material which facilitate theproduction of a spot weld.

What is claimed is:
 1. A method of securing a multilayer metal wire meshto a metal substrate comprising the steps ofproviding a metal substratewith a plurality of step-like protuberances on an outer surface thereof;positioning a multilayer metal mesh over said outer surface andprotuberances of the substrate; pressing the metal mesh against thesubstrate to permanently deform discrete sections of the mesh disposedover at least some of the protuberances substantially withoutpermanently deforming the remainder of the mesh; and welding thepermanently deformed sections to the protuberances and the layers of thedeformed sections of the multilayer mesh to each other therein.
 2. Amethod as set forth in claim 1 wherein said step of welding productsspot welds between the mesh layers in the permanently deformed sectionsand between the protuberances and the permanently deformed sections. 3.A method as set forth in claim 1 wherein said steps of pressing andwelding are performed simultaneously.
 4. A method as set forth in claim3 wherein said steps of pressing and welding employs a mesh-compressingram with spot-welding electrodes therein.